Dual ramp analog to digital converters are extremely accurate high performance systems. Basically, the approach of integrating a voltage in a first direction for a predetermined time period during the first half of the measuring cycle and then removing charge or integrating in an opposite direction at a predetermined rate during the second phase of the measuring cycle permits supply voltages and device tolerance variations to offset each other. In this manner, the net effect of these tolerance variations is essentially zero and thus accuracy is improved.
Many improvements have been suggested for the basic dual ramp analog to digital converter in order to improve performance and accuracy. However, a major problem still exists in the implementation of a dual ramp analog to digital converter which can economically and efficiently handle unknown analog voltages of positive and negative polarity. The conventional approach is to employ a dual supply system. In any type of implementation errors are introduced if both supplies vary or do not track with respect to each other. Moreover, although some other approaches or solutions are satisfactory in a non-integrated circuit environment, the implementation of a dual ramp analog to digital converter in an integrated circuit poses problems associated with calibration and auto-polarity and single supply capability.